Bibliographic details of the publications referred to by author in this specification are collected alphabetically at the end of the description.
Reference to any prior art in this specification is not, and should not be taken as, an acknowledgment or any form of suggestion that this prior art forms part of the common general knowledge in any country.
An increasing human population necessitates improvements in crop productivity. This has been a major goal for plant breeders and geneticists. One approach to improving crop productivity involves the selection of plant traits which facilitate higher grain yield and stability (Springer (2010) Nature Genetics 42:475-476). This approach has been referred to as the “Green Revolution”. Other approaches include the development of ideal plant architectures which have, for example, led to the identification of a quantitative trait locus (QTL) which encodes squamosa promoter binding protein-like 14 (OsSPL14) in rice and which facilitates improved rice yield (Jiao et al. (2010) Nature Genetics 42:541-544; Miura et al. (2010) Nature Genetics 42:545-549).
Drought is the single most important constraint to cereal production worldwide. Sorghum is a repository of drought resistance mechanisms, which include C4 photosynthesis, deep roots and thick leaf wax which enable growth in hot and dry environments. Drought tolerance makes sorghum especially important in dry regions such as sub-Saharan Africa, western-central India, north-eastern Australia, and the southern plains of the US. With increasing pressure on the availability of scarce water resources, the identification of traits associated with grain yield under drought conditions becomes more important.
The drought adaptation mechanism identified in sorghum which results in the retention of green leaves for longer periods during grain filling under drought is known as ‘stay-green’. Stay-green has been associated with high grain yield under post-anthesis drought in sorghum (Borrell et al. (200b) Crop Sci. 40:1037-1048; Kassahun et al. (2010) Euphytica 72:351-362), wheat (Triticum aestivum L.) [Spano et al. (2003) J. Exp. Bot. 54:1415-1420; Christopher et al. (2008) Aust. J. Agric. Res. 59:354-364], rice (Oryza sativa L.) [Kashiwagi et al. (2006) Plant Physiology and Biochemistry 44:152-157] and maize (Zea mays L.) [Zheng et al. (2009) Plant Breed 128:54-62]. In addition, it may indirectly affect grain yield under drought by improving charcoal rot (Macrophomina phaseolina [Tassi] Goid.) resistance (Tenkouano et al. (1993) Theor. Appl. Genet. 85:644-648; Garud et al. (2002) Int. Sorghum and Millets Newsl. 43:63-65). This reduces lodging (Reddy et al. (2008) Euphytica 159:191-198), allowing plant breeders to exploit the positive association between plant height and grain yield (Jordan et al. (2003) Theor. Appl. Genet. 106:559-567). Stay-green has been an important selection criterion for sorghum breeding programs targeting drought adaptation in both the US (Rosenow et al. (1983) Agric. Water Manag. 7:207-222) and Australia (Henzell et al. (1997) Australia Int. Sorghum and Millets Newsl. 38:1-9).
A considerable body of physiological evidence is mounting in support of this trait (Borrell et al. (2000a) Crop Sci. 40:1026-1037; Borrell and Hammer (2000) Crop Sci. 40:1295-1307; Harris et al. (2007) J. Exp. Bot. 58:327-338; Christopher et al. (2008) supra; Van Oosterom et al. (2010a) Field Crops Res. 115:19-28 and Van Oosterom et al. (2010b) Field Crops Res. 115:29-38). Although this drought resistance mechanism has been utilized by sorghum breeders in the US and Australia for over 25 years, and the broad physiological basis of the trait is becoming better understood, the causal mechanisms and the genetic loci involved have hitherto been unknown.
Under water limiting conditions, grain yield is a function of transpiration (T), transpiration efficiency (TE), and harvest index (HI) [Passioura (1977) J. Aust. Inst. Agric. Sci. 43:117-120]. Within this framework, grain yield is linked to post-anthesis T (Turner (2004) J. Exp. Bot. 55:2413-2425; Manschadi et al. (2006) Funct. Plant. Biol. 33:823-837), because HI increases with the fraction of total crop T used after anthesis (Passioura, (1977) supra; Sadras and Connor (1991) Field Crops Res. 26:227-239; Hammer (2006) Agric. Sci. 19:16-22). Increased post-anthesis T is associated with reduced drought stress around anthesis, which can positively affect crop growth rate at anthesis of cereals and hence grain number (Andrade et al. (2002) Crop Sci. 42:1173-1179; Van Oosterom and Hammer (2008) Field Crops Res. 108:259-268). If the total amount of available water is limited, post-anthesis T can be increased by restricting pre-anthesis T. This can be achieved by restricting canopy size, either genetically or through crop management. However, a smaller canopy will only reduce total T if its TE is not compromised. Significant genotypic differences in TE have been reported for sorghum (Hammer et al. (1997) Aust. J. Agric. Res. 48:649-655; Henderson et al. (1998) Aust. J. Plant Physiol. 25:111-123; Mortlock and Hammer (1999) J. Crop Prod. 2:265-286; Xin et al. (2009) Field Crops Res. 111:74-80). Alternatively, post-anthesis water use can be increased by increasing the total amount of water accessed by the crop, either through deeper rooting or reduced lower limit of water extraction (Manschadi et al. (2006) supra).
The stay-green trait affects a number of the above processes in sorghum. First, stay-green reduces water use during the pre-anthesis period by restricting canopy size (via reduced tillering and smaller leaves).
Second, stay-green improves water accessibility by increasing the root:shoot ratio. There is some experimental evidence for better water extraction in stay-green lines, although more research is required. These root responses could also be explained by enhanced auxin transport (Wang et al. (2009) Molecular Plant 2(4):823-831). Third, stay-green increases the greenness of leaves at anthesis, effectively increasing photosynthetic capacity, and, therefore. TE (providing that photosynthesis increases proportionately more than conductance). The increase in leaf greenness is an indirect affect of reduced leaf mass, i.e. nitrogen is concentrated in the leaf.
Producing more food with less water is one of the major challenges currently facing humanity. There is a real and urgent need in both developing and developed countries to identify the genes and gene networks controlling drought adaptation in crop plants. This enables increased drought adaptation in a wide range of crop species grown in water-limited environments worldwide.